SixtyPical/eg/game.60p

assign byte table screen $0400
assign byte table screen2 1274
assign byte table screen3 1524
assign byte table screen4 1774

assign byte table colormap 55296
assign byte table colormap2 55546
assign byte table colormap3 55796
assign byte table colormap4 56046

assign byte vic_border 53280
assign byte table vic_bg 53281

assign byte joy2 $dc00

assign vector cinv 788

; ---------

reserve vector save_cinv

assign word position $fb
assign word new_position $fd

reserve word delta
reserve byte value
reserve word compare_target

routine reset_position {
    copy #$0400 position
}

routine advance_pos {
    clc
    lda <position
    adc <delta
    sta <new_position
    lda >position
    adc >delta
    sta >new_position
}

routine install_new_position {
    copy new_position position
}

routine compare_new_pos {
    lda >new_position
    cmp >compare_target
    if beq {
        lda <new_position
        cmp <compare_target
    } else {
    }
}

routine check_new_position_in_bounds {
    copy #$07e8 compare_target      ; just past bottom of screen
    jsr compare_new_pos

    if bcs {
        clc
    } else {

        copy #$0400 compare_target
        jsr compare_new_pos

        if bcc {
            clc
        } else {
            sec
        }
    }
}

routine clear_screen {
    ldy #0
    repeat bne {
        lda #1
        sta colormap, y
        sta colormap2, y
        sta colormap3, y
        sta colormap4, y
        
        lda #32
        sta screen, y
        sta screen2, y
        sta screen3, y
        sta screen4, y

        iny
        cpy #250
    }
}

routine read_stick {
    lda #0
    sta <delta
    sta >delta
    ldx joy2
    txa
    and #1                   ; up
    if beq {
        lda #216             ; -40
        sta <delta
        lda #255
        sta >delta
    } else {
        txa
        and #2               ; down
        if beq {
            lda #40
            sta <delta
        } else {
            txa
            and #4           ; left
            if beq {
                lda #255     ; -1
                sta <delta
                lda #255
                sta >delta
            } else {
                txa
                and #8       ; right
                if beq {
                    lda #1
                    sta <delta
                } else { }
            }
        }
    }
}

routine our_cinv {
    lda #32
    ldy #0
    sta (position), y
    jsr read_stick
    jsr advance_pos
    jsr check_new_position_in_bounds
    if bcs {
        jsr install_new_position
    } else { }

    lda #81
    ldy #0
    sta (position), y

    jmp (save_cinv)
}

routine main {
    lda #5
    sta vic_border
    lda #0
    sta vic_bg
    jsr reset_position
    jsr clear_screen
    sei {
        copy cinv save_cinv
        copy routine our_cinv to cinv
    }
    clc
    repeat bcc { }
}